The present invention relates to an improved die head assembly, apparatus, and process for meltblowing a thermoplastic polymer to form a fiber and a nonwoven fabric web, and more particularly relates to an improved die head assembly, apparatus, and process for meltblowing a fiber-forming thermoplastic polymer using an air flow through a removably securable passageway that is centrally-located within an extruded polymer flow, or using an extruded polymer flow through an elongated opening, to form meltblown fibers and a nonwoven fabric web.
Meltblowing techniques for forming very small diameter fibers, sometimes referred to as microfibers or meltblown fibers, from thermoplastic resins and polymers are well-known in the art. For example, the production of fibers by meltblowing is described in an article entitled "Superfine Thermoplastic Fibers", appearing in Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346. This article describes work done at the Naval Research Laboratories in Washington, D.C. Another publication dealing with meltblowing is Naval Research Laboratory Report 111437, dated Apr. 15, 1954. Generally, meltblowing techniques include heating a thermoplastic fiber-forming resin to a molten state and extruding the molten resin from a die arrangement having a plurality of linearly arranged small diameter capillaries as molten threads. The molten threads exit the die into a high velocity stream of gas, usually air, which is maintained at an elevated temperature, and which serves to attenuate the threads of molten resin to form fibers having a diameter which is less than the diameter of the capillaries of the die arrangement.
A typical apparatus and process for forming a meltblown fabric is shown in FIG. 1, in which a hopper 10 provides polymer material to an extruder 12 attached to a die 14 which extends across the width 16 of a nonwoven web 18 to be formed by the meltblowing process. Inlets 20 and 22 provide pressurized gas to die 14. FIG. 2 shows a partial cross-section of a portion of die 14, including an extrusion slot 24 that receives polymer from extruder 12 and chambers 26 and 28 that receive pressurized gas from inlets 20 and 22. Chambers 26 and 28 are defined by base portion 30 and plates 32 and 34 of die 14.
The melted polymer is forced out of slot 24 through a plurality of small diameter capillaries 36 extending across tip 38 of die 14. Capillaries 36 have a diameter on the order of 0.0145 to 0.0180 in., and are spaced from 9-30 capillaries per inch. The gas passes from chambers 26 and 28 through passageways 40 and 42. The two streams of gas from passageways 40 and 42 converge to entrain and attenuate molten polymer threads 44 (see FIG. 1) as they exit capillaries 36 and land on a foraminous surface 46, such as a belt. The molten material is extruded through capillaries 36 at a rate of from 0.02 to 1.7 grams/capillary/minute at a pressure of up to 300 p.s.i.g. The temperature of the extruded molten material is dependent on the melting point of the material chosen, and is often in the range of 125 to 335.degree. C. The gas may be heated to 100 to 400.degree. C. and pressurized up to 20 p.s.i.g.
The extruded threads 44 form a coherent, i.e. cohesive, fibrous nonwoven web 18 that may be removed by rollers 47, which may be designed to press web 18 together to improve the integrity of web 18. Thereafter, web 18 may be transported by conventional arrangement to a wind-up roll, pattern-embossed, etc. U.S. Pat. No. 4,663,220 discloses in greater detail an apparatus and process using the above-described elements, and is incorporated by reference herein.
U.S. Pat. No. 4,818,464, the disclosure which is also incorporated by reference herein, discloses a process and apparatus for meltblowing thermoplastic material using a different type of die head. In this patent, a centrally located gas jet passes through or between an opening or openings for extruding thermoplastic material. FIG. 3 shows a partial sectional view of a die tip as taught in U.S. Pat. No. 4,818,464. As shown, gas inlet 48 and extrusion openings 50 and 52 are arranged such that the longitudinal axes 54 and 56 of openings 50 and 52 are disposed at an angle with longitudinal axis 58 of inlet 48 of about 30 degrees to less than about 90 degrees, and typically about 60 degrees. (See angles 60 and 62).
While the above devices work well for their intended purposes, they are subject to a few minor drawbacks. For example, the small diameter capillaries used to deliver molten thermoplastic material in the above devices require very precise machining to properly locate and create the capillaries. Thus, die heads with extremely fine capillaries are expensive to create.
Also, the small diameter capillaries may be clogged if, for example, molten thermoplastic material were to char or degrade prior to reaching the capillary, forming a solid particle too large to fit through the capillary. Further, the addition of pigments and other additives, or the presence of impurities, could similarly cause clogging of capillaries. It is a time-consuming and expensive process to halt production of a meltblowing line, remove the partially clogged die head assembly, install a clean die head assembly, and clean the clogged die head assembly capillaries for future use.
Further, the small diameter capillary may require the molten thermoplastic material to be heated to an extremely high temperature in order to ensure a low enough viscosity to allow for smooth flow through the small diameter capillary. Also, a high pressure must be used to ensure the molten thermoplastic material is properly extruded through the small diameter capillaries at a flow rate high enough to justify commercial production. With the higher pressures and temperatures used, the entire apparatus must be larger and the energy usage must be higher. Alternately, a low molecular weight (high meltflow rate) material may be required to achieve a suitable low viscosity. Such low molecular weight polymers are often more expensive than materials than have not been treated to attain these characteristics.
Moreover, due to the small diameter of the capillaries, it is difficult to give the capillary any shape other than a circle. Thus, the shape of the fibers formed from such a die head assembly are limited to those attainable by extrusion through a circular capillary.
Also, typical commercial die head assemblies include one row of capillaries, rather than a large array of rows of capillaries, because of the need to have intimate contact between the primary air and polymer filaments to properly attenuate the filaments. The prior art does not permit the spacing of multiple rows of capillaries adjacent each other. Thus, commercial production is limited to the amount of polymer that can be extruded from a single row of extremely small capillary openings. In order to make commercial production feasible, high polymer velocity must be achieved through each hole, generally leading to larger fibers and/nor harsher webs, which can be undesirable.